Bridging the surfaces of two solid materials is a long-standing yet critical topic in many engineering fields. Bonding two solids at their interface is the most effective way to achieving a mechanically robust and thermally conducting interface. However, for high temperature applications, bonded interfaces between dissimilar materials experience high thermomechanical stress that degrades their performance, cyclic stability (under thermal load) and lifetime. Today, from macroscale applications in welded joints to micro and nanoscale applications in composites and nanoelectronic devices, interfaces play a crucial role in overall system performance and are often the limiting factor. In particular, miniaturization of electronic devices has placed much emphasis on development of interfaces that can sustain high thermal and electronic transport. While much effort has been directed towards improving interfacial bonding for enhanced thermal or electronic transport at or around room temperature, high temperature applications still pose a serious challenge.
For most material systems, interfacial bond strength and thermal transport are strongly correlated. For instance, load-bearing joints typically bonded firmly by welding, brazing or soldering, also yield the lowest thermal resistance (1-5 mm2K/W). A weaker joint made, for example, with polymeric adhesives provides moderate thermal resistance (10-200 mm2K/W). In comparison, a bare interface between two solids without any interfacial bonding has no mechanical strength and also the highest thermal resistance (>100 mm2K/W). Thus, generally a strongly bonded interface has a relatively low interfacial thermal resistance. However, for applications operating under high (and often cyclic) thermal load (e.g. a thermoelectric generators, a solid oxide fuel cells and plasma-facing components in fusion reactors), a stable and strongly bonded interface is not an easy objective to achieve. The difficulty arises from the fact that strongly bonded interfaces generally also restrict relative movement of the two mating surfaces and therefore experience enormous thermomechanical stress at high temperatures, leading sometimes even to failure at the joint. This mechanical failure also then disrupts the thermal transport. The challenge for a high temperature thermal interface material thus lies in fulfilling the conflicting requirements of a strongly bonded interface that can also sustain relative motion between the surfaces.
Carbon nanotubes (CNTs) are nano-materials that are being intensely investigated as thermal interface materials due to their high thermal conductivity. In spite of the high thermal conductivity of an individual carbon nanotube (CNT), use of vertically oriented CNT arrays as a thermal interface material has been hindered by the high contact resistance at their free ends. Conventional methods to decrease the contact resistance often involve multi-step functionalization processes and suffer from poor uniformity. In addition, such methods typically require use of organic compounds or low-temperature solder, rendering the thermal interface material unsuitable for high temperature applications. Accordingly, there remains a need for further contributions in this area of technology.